Cathode-ray screen coated with zinc or magnesium fluoride phosphors



Patented June 26, 1951 'CATHODE-RAY SCREEN COATED WITH ZINC OR FLUORIDE PHOSPHORS Maggie 'C. 'Banca, Camden, N. J., assignor to .Radioflorporation of America; a corporation of Delaware I No Drawing. implication April 19, 1945,

Serial No. 589,269

Claims. I

My invention relates to luminescent materials and more particularly to improved luminescent materials or phosphors having visible phosphorescence persisting for an appreciable time and decaying in an unusual way that'makes the phosphor more useful for certain purposes than previously known phosphors.

Many phosphors which fluoresce when excited by energy, such as from electron bombardment, emit light or phosphoresce after cessation of excitation. The light emission due to phosphorescence is at a maximum at the instant of cessation of fluorescence and then decays in a way dependent upon the persistence of the particular phosphor until the light becomes invisible.

Phosphors may be roughly divided intotwo main classes in reference to the phosphorescence decay characteristics. The more useful phosphorescent materials in one class have decay curves which follow a power law approximating the equation,

where L is the luminescence intensity at any time t, L0 is the intensity of luminescence when t=0 (i. e., at cessation of excitation), and b is a constant (usually of the order of In the second class of exponential decay phosphors, decay is substantially according to the equation,

where the constant a is of the order of 100 or larger and e is the Naperian logarithmic base.

For many purposes, such as in position finders or radars having scan times of the order of 0.1 to 0.5 second per scan, the flicker from phosphors decaying according to Equation 1 is very bad, whereas in those having the exponential decay form according to the Equation 2 the flicker is less. The large value of the constant a for useful exponential-decay phosphors heretofore known has, however, afforded too short a persistence to be useful for scan intervals longer than the persistence of vision of the human eye (about g second). It is therefore an object of my invention to provide a phosphor with an exponential decay characteristic in which the decay time is considerably greater than known phosphors.

Another object is to provide a phosphor adapted to phosphoresce after cessation of excitation for a much longer time than usual and at a higher light level with relatively sudden decrease to invisibility thereafter.

Still other objects will appear in the following description.

I have devised exponential-decayphosphorsin which the value of. the decay constant a in Equation 2 is only one-tenth as large as the best materials previously used. These materials have useful decay times about ten times as long as willemite, for example.

My improved material phosphoresces at high light level that is visible for a period around 0.5 second after cessation of excitation and then suddenly blanks out completely below visibility. This gives a minimum of carry-over from one image scan to the next in a cathode beam tube and obviates the blurring and trailing of moving objects observed on the usual long persistence screens. The phosphor is therefore useful in position and distance finding, oscillography and other uses in which the image-repetition interval is such that flicker is deleterious.

The phosphor of my invention comprises manganese activated zinc fluoride, or magnesium fluoride, or combinations of the two, either as mechanical mixtures or as co-crystallized com pounds.

As one example, a phosphor embodying my invention, with which I have obtained good results, was made by treating twenty grams of chemically pure zinc oxide with concentrated aqueous hydrofluoric acid in excess, adding 0.1 gram of manganese chloride, mixing, drying and heating to about 700 0., in a platinum crucible, for about twenty minutes. The product is preferably reground and heated at 700 C. until excess hydrofluoric acid is driven off. The reheated product is then carefully ground to fine powder, such as by ball milling. The cinder obtained by the heating is white and soft. This proportion of activator is about 0.17% of the zinc content.

The fluorescent screen containing the material can be made in conventional ways, but it should be noted that fluoride phosphors should not be exposed to moisture and non-aqueous liquids should be used if the material is settled on a screen from a liquid, as known in the art.

In use, the phosphor has a brilliant yellow fluorescence and phosphorescence with a subjective value by visual test of 7 to 8 with respect to a scale where willemite is 10, and with long time lag, at least twice that of standard willemite and in which the bright phosphorescence ceases very abruptly and completely with no visible afterglow.

I have prepared an eflicient magnesium fluoride phosphor by following the method as described in reference to zinc fluoride, for example, using five grams magnesium oxide and 0.05

manganese chloride with aqueous hydrofluoric acid. This phosphor gives a yellowish phosphorescence of a value of 6 or 7 and has a considerably longer time lag than that of willemite. This proportion of activator is about 0.28% of magnesium content.

My improved materials also phosphoresce after previous excitation, by heating, and by friction, with the same yellow color as when excited by cathode ray bombardment.

Having described my invention, what I claim 1. A cathode ray screen having thereon phosphor material consisting of a fluoride of a metal from the group consisting of zinc and magnesium, and manganese activator in activator propore tions.

2. A cathode ray screen having thereon phos- 4 tivator being about 0.17% of the weight of the zinc fluoride.

5. A cathode ray screen having thereon phosphor material consisting of magnesium fluoride and manganese activator, the manganese in said activator being about 0.28% of the weight of the magnesium fluoride.

MAGGIO' C. BANCA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,049,765 Fischer Aug. 4, 1936 2,372,071 Fernberger Mar. 20, 1945 FOREIGN PATENTS Number Country Date 306,978 Italy Apr. 6, 1933 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. IV, p. 296.

Chemical Abstracts, vol. 18, p. 2287, 1924. 

1. A CATHODE RAY SCREEN HAVING THEREON PHOSPHOR MATERIAL CONSISTING OF A FLUORIDE OF A METAL FROM THE GROUP CONSISTING OF ZINC AND MANGNESIUM, AND MANGANESE ACTIVATOR IN ACTIVATOR PROPORTIONS. 